1. Field of the Invention
The present invention relates to a method for supplying rolling oil for cold rolling and, more particularly, to a method for supplying rolling oil in a recirculation mode in cold tandem rolling using a plurality of rolling mill stands.
2. Description of Related Arts
In cold rolling, lubrication is needed to decrease friction between a steel sheet and a roll during rolling, and also the roll and the steel sheet must be cooled to remove frictional heat and working heat.
Usually, in cold rolling, lubrication is performed by using emulsion rolling oil. Emulsion is a mixed liquid in which particles of rolling oil exist in a state of being suspended stably in water, and is characterized by particle diameter and concentration. The particle diameter of emulsion means the particle diameter of rolling oil in emulsion, and the concentration of emulsion means the ratio of oil volume to total volume of emulsion. Emulsion is prepared by adding a predetermined quantity of emulsifier with a concentration with respect to the amount of rolling oil, and then by applying shearing by means of an agitator or a pump. The methods for supplying emulsion rolling oil in cold rolling involve a direct mode, a recirculation mode, and a hybrid mode which is a compromise between the former two modes.
In the direct mode, high concentration emulsion rolling oil is sprayed on a steel sheet for the purpose of lubrication, and water is sprayed on a roll for the purpose of cooling. Therefore, this mode has higher lubricating and cooling abilities than the recirculation mode. In this mode, however, since the emulsion rolling oil is not recirculated, the unit consumption of rolling oil is high.
On the other hand, in the recirculation mode, low concentration emulsion rolling oil, prepared in advance by mixing and agitating rolling oil and cooling water, is sprayed on a steel sheet and a roll for the purpose of lubrication and cooling while being recirculated. Therefore, the unit consumption of rolling oil is low. However, this mode has lower lubricating and cooling abilities than the direct mode, so that lubrication is insufficient at the time of high speed rolling, for example, in the case where a thin steel sheet with a thickness not larger than 0.2 mm is rolled, and also vibration of a rolling mill called chattering and surface defect called heat scratch are liable to occur.
In order to improve the lubricating ability in the method for supplying rolling oil in the recirculation mode, JP-59-24888 (the term xe2x80x9cJP-Bxe2x80x9d referred to herein signifies xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d) has disclosed a method in which rolling oil with a concentration not lower than 10% is sprayed directly on the back surface of a steel sheet at a rate of 100 to 200 l/min just before rolling. With this method, however, the lubricating ability is not improved fully for the following reasons i) and ii).
i) A phenomenon that oil in the sprayed emulsion sticks to the front surface of steel sheet is called plate out. The inventors conducted studies on the relationship between the ratio of the quantity of plate out oil to the quantity of oil contained in the supplied emulsion (hereinafter referred to as a sticking efficiency) and the concentration. As a result, it was found that if only the concentration of emulsion was increased, the sticking efficiency decreased, so that the plate out amount necessary for lubrication could not be obtained.
ii) At the time of high speed rolling, a heat scratch develops not only on the back surface but also on the front surface of the steel sheet.
JP-B-58-5731 and JP-A-9-122733 (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d) have disclosed a method of the hybrid mode in which apart from a rolling oil supply system for supplying low temperature emulsion with a low concentration lower than 10%, high concentration emulsion, which has been prepared by adding rolling oil to the extract of the recirculating emulsion, is heated to a high temperature of 75xc2x0 C., and is sprayed directly on a steel sheet just before rolling. With this method, however, not only the lubricating ability is not improved sufficiently, but also the equipment cost increases remarkably for the following reasons iii) and iv).
iii) Even if the temperature of emulsion is increased, the sticking efficiency is not always increased, depending on an emulsifying dispersant constituting the emulsion. Also, as described in item i), even if the concentration of emulsion is increased, a sufficient plate out amount is not obtained. Furthermore, an extra device such as a heater is needed to heat and pressurize the emulsion, and also equipment such as a large sized cooler is needed for the rolling oil supply system because the mixing of the high temperature emulsion for the rolling oil supply system raises the temperature.
iv) In the hybrid mode, when the recirculating emulsion is extracted, the characteristics such as ESI (Emulsion Stability Index), concentration of iron, saponification value, and acid value of the recirculating emulsion are not fixed, so that chemical treatment for making these characteristics fixed and complicated equipment such as a filter for removing iron are needed. Also, if the characteristics of the extracted emulsion change once, the characteristics of the emulsion of the hybrid mode change accordingly.
JP-A-2-37911 has disclosed a method in which an emulsifier added to the high concentration emulsion is of the same kind as that of the recirculating emulsion, and only the quantity of added emulsifier is changed to decrease the ESI, by which the emulsion is sprayed directly on a steel sheet. With this method, however, emulsion of the hybrid mode, which has a low ESI and unstable emulsifying dispersibility, is included in a rolling oil supply tank, so that the ESI of the recirculating emulsion changes with time and the emulsifying dispersibility becomes unstable. Therefore, excessive lubrication occurs, and slip occurs at all rolling mil stands, whereby rolling becomes unstable.
JP-B-63-5167 has disclosed a method in which using an ionic emulsifier, which does not yield salt or is less liable to yield salt, emulsion with unstable emulsifying dispersibility is sprayed directly on a steel sheet, and then the emulsion is included in a rolling oil supply tank to which a water soluble negative ionic substance is added excessively, by which emulsification is stabilized. Since the rolling oil supply tank of a lubrication system generally has a high capacity of about 10,000 liters, in this method, a large quantity of negative ionic substance must be added in advance into the tank. Therefore, the emulsifying dispersibility of emulsion in the tank varies easily. Also, when an ionic emulsifier less liable to yield salt is used, a salt yielding reaction with emulsion is difficult to take place, so that the emulsion in the tank becomes unstable.
JP-B-6-13126 has disclosed a method in which the quantity of supplied emulsion is controlled to establish a proper lubrication state. With this method, however, since the change range of forward slip ratio is narrow, it is difficult to control the quantity of supplied emulsion so as to obtain the target forward slip ratio, so that a proper lubrication state cannot be established.
The present invention has been made to solve the above problems with the prior art, and accordingly an object thereof is to provide a method for supplying rolling oil for cold rolling in a recirculation mode which can achieve sufficient lubricating ability, capable of preventing the occurrence of chattering and heat scratch even at the time of high speed rolling, steadily at a low cost.
To achieve the above object, the present invention provides a method for supplying rolling oil for cold rolling wherein emulsion rolling oil is supplied using both: (i) a first rolling oil supply system for supplying the emulsion rolling oil to a roll and a steel sheet being rolled in a recirculation mode, and (ii) a second rolling oil supply system for supplying the emulsion rolling oil to only the front and back surface of the steel sheet being rolled. The emulsion rolling oil supplied by the first and second rolling oil supply systems comprise an emulsifier of a same type and concentration, and the second rolling oil supply system is controlled so that an average particle diameter of emulsion is larger than an average particle diameter of emulsion of the first rolling oil supply system. The emulsion of the second rolling oil supply system, which has not stuck onto the steet sheet being rolled, is the joined to the emulsion of the first rolling oil supply system.
Also, to achieve the above object, the present invention provides another method for supplying rolling oil for cold rolling wherein emulsion rolling oil is supplied using both: (i) a first rolling oil supply system for supplying the emulsion rolling oil to a roll and a steel sheet being rolled in a recirculation mode, and (ii) a second rolling oil supply system supplying the emulsion rolling oil to only the front and back surface of the steel sheet being rolled. According to this method, the emulsion rolling oil supplied by the second rolling oil supply system comprises an emulsifier of a same type but lower concentration as an emulsifier of the first rolling oil supply system, and the second rolling oil supply system is controlled so that an average particle diameter of emulsion is larger than an average particle diameter of emulsion of the first rolling oil supply system. The emulsion of the second rolling oil supply system which has not stuck onto the steel sheet being rolled is then recovered, additional emulsifier is added to the recovered emulsion so that the concentration of the emulsifier is equal to that of the emulsifier of the first rolling oil supply system, and the emulsion of the second rolling oil supply system is mechanically agitated. The emulsion of the second rolling oil supply system, having been agitated mechanically, is then joined to the emulsion of the first rolling oil supply system.